1. Field of the Invention
The present invention concerns an electroconductive magnetic fluid composition provided with antistatic properties, and a process for producing the same.
2. Description of the Prior Art
Since magnetic fluids generally exhibit high electrical resistance, when they are used, for example, as the sealing mechanism for magnetic disc devices, etc., it has been the general practice to incorporate electrical grounding means into such devices for eliminating static charges accumulated on the magnetic disc devices, etc. (hereinafter simply referred to as electrified body). In view of the above, electroconductive magnetic fluids have been proposed in the prior art which are capable of preventing static charges without the use of such grounding means, by providing electroconductivity to the magnetic fluid per se (refer to U.S. Pat. No. 4,604,222 and Japanese Patent Laid-Open No. Sho 610274737).
An organic solvent such as mineral oil or poly-alpha-olefin oil is used as a carrier in some of the prior art, and an anionic surface active agent is used for stably dispersing fine ferromagnetic particles in the carrier in the usual magnetic fluids. A cationic surface active agent, for example, a quarternary ammonium salt represented by the structural formula: ##STR1## where X represents a halogen and R.sub.1 -R.sub.4 each represent a hydrocarbon chain, is used in the prior art for forming a coating layer or a second coating layer on fine ferromagnetic particles.
The prior art cationic surface active agent comprises a polar cationically charged portion, and a long-chained nonpolar portion which is mutually soluble in the carrier. The surface of the fine ferromagnetic particles is coated with the surfactant, the positively charged portion of the surfactant being electrostatically absorbed to the particle surface, and the long-chained portion of the surfactant being directed toward the surrounding carrier. The magnetic particles are thereby stably dispersed in the carrier and the electroconductivity of the magnetic fluid is improved. Accordingly, it is possible to use this prior art electroconductive magnetic fluid, for example, as a sealing agent for a disc driving device, so that static charges, which would otherwise tend to be accumulated on the disc, can be removed to attain antistatic performance.
However, the conventional electroconductive magnetic fluids as described above have the following problems:
(1) Since each of the magnetic particles are coated with the cationic surface active agent as a charged body, the magnetic particles tend to be moved under the effect of the charge possessed by the electrified body towards that opposite charge. This tends to make the distribution of the particle concentration not uniform in the magnetic fluid. Accordingly, when the electroconductive magnetic fluid is used, for example, as a sealing agent, the saturation magnetization thereof is reduced where the concentration of the magnetic particles is low, which may even lead to the destruction of the sealing oil membranes and resultant deterioration of the sealing performance. PA1 (2) When the electric charges on the electrified body are offset with the cationic surface active agent, the cationic surface active agent tends to be easily detached from the surface of the fine ferromagnetic particles and, accordingly, satisfactory dispersion of the fine ferromagnetic particles is harder to achieve in the magnetic fluid. PA1 (3) The cationic surface active agent serves both for dispersing the ferromagnetic particles and providing electroconductivity to the fluid. Accordingly, the amount of the surfactant added is inevitably limited by the concentration of the fine ferromagnetic particles and, thus, the quantity of the saturation magnetization, making it difficult to independently control the electroconductivity of the solution. PA1 (4) Since a cationic surface active agent of poor heat resistance is used in the prior art, the surface active agent is decomposed or evaporated at high temperature with elapse of time. Accordingly, the electroconductivity of the magnetic fluid conditioned by adding the surface active agent is gradually lowered as the surfactant is lost. PA1 (a) adding fine ferromagnetic particles to a low boiling point organic solvent and a surface active agent having a lipophilic group which is mutually soluble therein for coating the surface of the fine ferromagnetic particles, thereby obtaining an intermediate medium in which the fine ferromagnetic particles are coated at the surface thereof with the surface active agent and are uniformly dispersed in the low boiling point organic solvent; PA1 (b) separating out any fine particles of poor dispersibility in the intermediate medium and, thereafter, adding a less volatile organic solvent to the intermediate medium to form a mixture; PA1 (c) heating the mixture to evaporate and separate the low boiling point organic solvent to obtain a magnetic fluid; and PA1 (d) adding a mixture comprising a tertiary amine and a fatty acid to provide electroconductivity to the resultant magnetic fluid. PA1 (a) adding a low boiling point organic solvent and a surface active agent having a lipophilic group which is mutually soluble therein to fine ferromagnetic particles to bond the surface active agent to the surface of the fine ferromagnetic particles; PA1 (b) thereafter, removing the low boiling point organic solvent to obtain fine ferromagnetic particles coated at the surface thereof with the surface active agent; PA1 (c) mixing the fine ferromagnetic particles with a low volatility organic solvent and a mixture comprising a tertiary amine and a fatty acid; and PA1 (d) removing any fine particles of poor dispersibility from the mixture.
The antistatic agent generally utilized so far for synthetic fibers or synthetic resins includes quarternary ammonium salts i.e., cationic surface active agents, as well a tertiary amines as nonionic surface active agents, e.g., N. N-bis(2-hydroxyethyl)aliphatic amine: ##STR2## where m, n each represents an integer of 1 or greater and R represents an aliphatic hydrocarbon chain. However, prior art compounds generally have poor heat resistance and tend to decompose at a high temperatures with elapse of time, and this tends to result in a reduction of the antistatic properties of the fluid.